The Function of Chemical Senses

“Gatekeepers” of the body: • Identify things that should be consumed for survival • Detect things that would be harmful and should be rejected • Cause good and bad affective responses

Basic Taste Qualities

Salty Sour Sweet Bitter Umami: described as meaty, brothy or savory, and associated with monosodium glutamate (MSG)

Sweet - Associations Between Taste Quality and a Substance’s Effect

usually substances that have nutritive value no perfect connection between tastes and function of substances

Bitter - Associations Between Taste Quality and a Substance’s Effect

usually substances that are potentially harmful no perfect connection between tastes and function of substances

Salty - Associations Between Taste Quality and a Substance’s Effect

presence of sodium no perfect connection between tastes and function of substances

Taste Map

All qualities of taste can be elicited from all the regions of the tongue that contain taste buds It has been known for many years that the concept of the “taste map” is incorrect

Tongue has four types of papillae

Filliform (no taste buds) Circumvallate (50% tb) Foliate (25% tb) Fungiform (25% tb)

Structure of Taste System

Taste buds are located in papillae except for filiform. Tongue contains approximately 10,000 taste buds. Each taste bud has 50-100 taste cells with tips that extend into the taste pore. Transduction occurs when chemicals contact the receptor sites on the tips.

Pathways for signals from taste cells

Chorda tympani nerve from front/sides of tongue Glossopharyngeal nerve from back of tongue Vagus nerve from mouth and throat Superficial petronasal nerve from soft palate

Pathway to brain

nucleus of solitary tract (brain stem) → thalamus → frontal lobe (Insula, Frontal operculum cortex, Orbital frontal cortex)

Population Coding, Erickson (1963)

Different taste stimuli were presented to rats, and recordings were made from chorda tympani. Across-fiber patterns showed that two substances (ammonium chloride and potassium chloride) are similar to each other but different from sodium chloride

Specificity Coding

Applying amiloride to the tongue blocks flow of sodium to taste receptors. Causes decrease in the responding of neurons in rat’s brainstem that respond most strongly to salt, but not to those that respond to a combination of salty and bitter tastes

Specificity or population Coding

Evidence exists for both specificity coding and population coding but balance of evidence is shifting toward specificity coding. Some researchers suggest that neural system for taste may function like visual system for color. Currently no agreed upon explanation for neural system for taste

Individual Differences in Taste

Different responses to phenylthiocarbamide (PTC) and to 6-n- propylthiouracil (PROP): • Tasters, nontasters, and supertasters • Tasters have more taste buds than nontasters. • Tasters have specialized receptors for these compounds. • Supertasters appear more sensitive to bitter substances than tasters.

Functions of Olfaction

Many animals are macrosmatic: having a keen sense of smell that is necessary for survival Humans are microsmatic: less keen sense of smell that is not crucial to survival Singh and Bronstad: showed a relationship between men’s rating of women’s body odors and women’s menstrual cycle

Detecting Odors

Rats are 8 to 50 times more sensitive to odors than humans. Dogs are 300 to 10,000 times more sensitive. However, individual receptors for all these animals equally sensitive. Difference: number of olfactory receptors (Humans 10M, Dogs 1B).

Identifying Odours

Humans can discriminate more than one trillion different odours. • Find it difficult to identify odours • Only successful half of the time

The Puzzle of Olfactory Quality

It has proven difficult to map perceptual experience onto physical attributes of odourants. • No specific ‘language’ for odour quality. • Some molecules that have similar structure smell different, and some that have different structures smell the same. • Links found between structure of molecules, olfactory quality, and patterns of activation in olfactory system.

The Olfactory Mucosa

located at top of nasal cavity. • Odourants carried along mucosa contact olfactory receptor neurons (ORN). • These neurons contain molecules called olfactory receptors (which allow a specific protein to cross membrane). • Humans: 350 receptor types

How Olfactory Receptor Neurons Respond to Odourants

Calcium imaging method Calcium concentration inside ORN increases when olfactory receptor responds. Calcium detected by using chemical that makes neuron fluoresce. Measuring decrease in fluorescence indicates strength of response.

Combinatorial code for odor (Malnic et al.) ORN responses

Odourants coded by patterns of activation of olfactory receptors (“recognition profiles”). Molecules with similar structures but different smell have different recognition profiles.

Optical imaging method - Search for Order in Olfactory Bulb

Cortical cells consume oxygen when activated. Red light is used to determine the amount of oxygen in the cells. Less oxygen reflects less red light. Measuring amount of light reflected reveals cortical activity

2-deoxyglucose (2DG [=radioactive]) technique - Search for Order in Olfactory Bulb

2DG injected, which contains glucose. • Animal exposed to different chemicals. • Neural activation is measured by amount of radioactivity present. • This technique, used with behavioral testing, shows pattern of neural activation is related to both chemical structure and to perception

Cortical Representation of Odours

Signals from olfactory bulb are sent to: Primary olfactory (piriform) cortex (temporal lobe) Then to secondary olfactory cortex (orbitofrontal cortex in frontal lobe) All interact with amygdala (in emotional reactions to odours)

Representation of Odourants in Piriform Cortex, Rennaker et a

used multiple electrodes to measure neural responses in piriform cortex isoamyl acetate causes (widespread) activation across cortex

How Odour Objects Are Represented/Learned

Wilson measured neural response in rat’s piriform cortex to 2 odourants • A mixture: isoamyl acetate and peppermint • A compound: isoamyl acetate alone With enough exposure, piriform cortex could discriminate between mixture and compound. Effect not found in Olfactory bulb.

Perception of Flavour

Combination of smell, taste, and other sensations (such as burning of hot peppers) Odor stimuli from food in mouth reaches olfactory mucosa through retronasal route. Taste of most compounds influenced by olfaction, but a few (e.g., monosodium glutamate) are not.

Taste and Olfaction Meet in Nervous System

Responses from taste and smell are first combined in orbital frontal cortex (OFC) Interactions between taste, olfaction, vision and touch underscore multimodal nature of our experience of flavour. Bimodal neurons: responding to more than one sense (typically similar qualities, e.g., taste and smell of sweet fruits) Not only taste and smell, but also texture, temperature, colour, sound, etc.

Flavour Is Influenced by Cognitive Factors

Plassmann experiment: Judging taste pleasantness of wine

Infant Chemical Sensitivity

Steiner: newborns can smell/discriminate between different olfactory stimuli. • Newborns can discriminate sweet, sour, and bitter stimuli • Mennella experiment: used combination of carrot juice and water to study infant preferences